Reliable optical communication systems require mechanisms for minimizing the effects of signal degradation occurring between associated transmitters and receivers. Signal degradation occurs due to a variety of factors that cannot be completely eliminated, and is exacerbated by the long transmission distances and high optical channel counts required in many applications. Due to signal degradation, some transmitted data may be incorrectly interpreted at a receiver. If data is misinterpreted at a rate above that which is acceptable, the efficacy and viability of the system may be lost.
A widely used measure of system performance is the system bit error rate (BER). The BER for a system is the ratio of the number of incorrectly received bits to the total number of received bits. For example, in a system having a BER of 10−9, one bit is misread out of every one billion bits received.
System performance may also be described in terms of the parameter Q. The Q value of a system indicates the signal-to-noise ratio of the electric signal regenerated by the receiver. Generally, when the signal to noise ratio or Q decreases, the bit error rate increases and vice versa. The specific relationship between the system Q value and BER depends on the particulars of the communication system. Typically, however, the BER rate improves exponentially with improvements in Q values above 10 dB.
In an effort to achieve acceptable system BER and Q, various advances in signal receiver configurations have been proposed. Typically, a receiver includes a single threshold decision circuit. The decision threshold circuit may, for example, include a comparator for comparing the received data signal with a predetermined voltage level (the decision threshold) at a predetermined time interval. If the voltage level of the received data signal is above the decision threshold at a particular sample time, the comparator may output a digital one. If, however, the voltage level of the received data signal is below the decision threshold, the comparator may output a digital zero.
The BER for a system including such a receiver is minimized by optimally setting the signal sampling time and threshold level. Unfortunately, however, even moderate amounts of signal distortion can limit the BER achievable by such receivers. These limitations are incompatible with the increase in data rates demanded by the ever-increasing capacity of optical networks.
Accordingly, there is a need for a signal receiver that overcomes the deficiencies of the prior art in providing reliable detection of a received data signal.